In recent years two distinct types of processes have been developed for desulfurizing gases by using molten alkali carbonate salt mixtures. One type of process scrubs a fuel gas (also known variously as reducing, synthesis gas, low or medium BTU gas, producer gas, town gas, etc.), whereas the other type of process scrubs a flue gas (also known as stack gas). Both types of processes are regenerative, i.e. there is no net consumption of alkali salt, and the sulfur ends up in salable form: elemental sulfur or sulfuric acid. Each type of process however entails characteristic disadvantages which have mitigated against their use as the desulfurization mechanism of conventional fossil fuel fired furnaces, e.g. electric utility boilers which burn coal, residual oil, and similar sulfurous carbonaceous fuels.
The flue gas approach is described in a series of U.S. patents assigned to and reports issued by Atomics International, including U.S. Pat. Nos. 3,438,722, 3,438,728, 3,551,108, 3,574,545, and the reports issued under EPA contract CPA 70-78 entitled "Development of a Molten Carbonate Process for Removal of SO.sub.2 from Power Plant Stack Gases". In essence the process is a three step process: SO.sub.2 in the flue gas reacts with the salt to form alkali sulfates plus minor amounts of sulfites; the sulfate in the salt is reduced to alkali sulfide by a reducing gas; and finally the sulfide is regenerated to carbonate by reaction with steam and CO.sub.2. The salt reduction step is incorporated because direct regeneration of a sulfate/sulfite salt by CO.sub.2 plus steam is thermodynamically extremely inefficient, compared to regeneration of sulfide salt. However, the salt reduction step has been found to also be highly inefficient, for a combination of reasons. First, there is a low utilization of the reducing gas-- equilibrium considerations dictate that there will be a high H.sub.2 plus CO concentration in the exhaust reducing gas exiting the reductor, plus a troublesome amount of H.sub.2 S. Secondly, the regenerated salt characteristically contains an appreciable residue of sulfide-- otherwise the regeneration would unavoidable be highly inefficient. This sulfide reacts with excess O.sub.2 in the flue gas to form sulfate, and that sulfate creates a demand for additional reducing gas. Thus this approach to desulfurization has never achieved commercial success due both to the high capital costs associated with the multiple step processing sequence and also due to the excessively high requirement for reducing gas or equivalent reducing agent (coke, electricity, etc.). U.S. Pat. No. 3,671,185 discloses a similar flue gas absorption step, but requires undesirably high temperatures, and also incorporates a solid phase salt regeneration process which is quite complex and inefficient.
The molten alkali carbonate salt approach to desulfurizing fuel gases is described in U.S. Pat. Nos. 3,919,390 and 3,996,335, and a series of reports by Battelle Pacific Northwest Laboratories entitled "Process for Removal of Sulfur Compounds from Low BTU Fuel Gases" including OCR (Office of Coal Research) R&D Report 100. The advantage of scrubbing a fuel gas vice flue gas is that alkali sulfide is formed directly during the scrubbing step. Thus no separate salt reduction step is necessary; the sulfide containing salt from the scrubber (also called absorber) can be sent directly to the regenerator. Regeneration of the sulfide to carbonate is accomplished by reaction with steam plus CO.sub.2 similarly to the corresponding step of the flue gas processes. The first intensive investigation of this regeneration step indicated that it would evidence a low efficiency (i.e. consume excessive amounts of steam plus CO.sub.2 per unit of H.sub.2 S scrubbed). However several recent disclosures have revealed methods of markedly increasing the regeneration efficiency for a given level of desulfurization. OCR R&D 100 discloses lowering the temperature of the salt prior to regeneration. Copending application 867,323 filed on Jan. 5, 1978 discloses use of countercurrent multistaging for both scrub and regeneration. By incorporating one or more of these techniques for increasing regeneration efficiency, it is now possible to regeneratively desulfurize a high temperature fuel gas both efficiently and economically. This capability however does not solve the needs of existing coal or oil fired furnaces. These furnaces incorporate the means for injecting and combusting the coal integrally with the boiler and related heat exchangers. Although it would be possible to add a separate fuel gas generator, desulfurize the fuel gas, and then inject the clean fuel gas into the boiler combustion zone, this would fail to utilize the existing coal firing capability while at the same time requiring the addition of an expensive fuel gas generator having its own coal firing capability. Such a solution appears marginal at best for existing furnaces, as distinct from new construction.
Another reason the existing molten alkali carbonate processes for fuel gas desulfurization do not meet the desulfurization needs of existing coal-fired boilers is that they must operate at relatively high temperatures, e.g. above approximately 1200.degree. F. Fuel gases contain high proportions of CO and high pCO/pCO.sub.2 ratios; thus cooling to that vicinity or lower can result in carbon deposition from the reverse Bouduard reaction. For fuel gas desulfurization processes there is a thermal efficiency advantage in operating at a temperature as close to the fuel gas generator temperature as possible, e.g. 1300.degree. to 1600.degree. F. However it would be very difficult to extract boiler flue gas in that temperature range, and other penalties would accrue even if it were done.
The term "combustion gas" as used herein encompasses both flue gas and fuel gas, i.e. it refers to the gas resulting from combustion (oxidation) of carbonaceous fuels regardless of the degree of completion of combustion.